1. Field of the Invention
The present invention relates to optical fiber telecommunication systems and, in particular, to an apparatus and method of manufacturing a blockless optical multiplexing device employed in such telecommunication systems.
2. Technical Background
The high-cost of installing new fiber-optic cable in order to increase the transmission capacity of an existing fiber-optic telecommunication system has given rise to the widespread use of optical multiplexing devices. Such optical multiplexing devices increase transmission capacity of a single fiber-optic waveguide by employing optical multiplexing techniques, such as, wavelength division multiplexing (WDM). WDM allows multiple different wavelengths to be carried over a common fiber-optic waveguide. Presently preferred wavelength bands for fiber-optic transmission media include those centered at 1.3 micrometer and 1.55 micrometer. The latter, with a useful bandwidth of approximately 10 to 40 nm depending on the application, is especially preferred because of its minimal absorption and the commercial availability of erbium doped optical fiber amplifiers. WDM can separate/divide this bandwidth into multiple channels. One particular technique of WDM referred to as dense wavelength division multiplexing (DWDM) divides this bandwidth into multiple discreet channels, such as 4, 8, 16 or even 32 channels. By combining and transmitting multiple signals simultaneously at different wavelengths over a single optical fiber transmission line, DWDM in effect, transforms one optical fiber into multiple virtual optical fibers, thus, increasing bandwidth over existing fiber-optic networks and providing a relatively low cost method of substantially increasing telecommunication capacity. One key advantage of a DWDM-based network is that it can transmit different types of traffic/data at different speeds over an optical channel. Accordingly, DWDM-based networks provide an efficient and cheaper way to quickly respond to customers"" bandwidth demands and protocol changes.
A prior art optical multiplexing device 200 is shown in FIG. 1. An essential optical component employed in such optical multiplexing devices 200 is an optical bandpass filter 132. As shown in FIG. 1, typically, two or more optical filters 132 are joined together to separate light of different wavelengths transmitted down a common optical waveguide. At a minimum, at least two optical filters are attached adjacent each other on an optical block 120 (as shown in FIG. 1) that has an optical slot 108 passing through the body of the optical block 120, where a collimated beam of light 118 passes through the optical slot 108 of the optical block 120 to each of the optical filters 132. Each of the optical filters 132 transmit light having a different predetermined wavelength and reflects light having other wavelengths. The optical block 120 is made of ceramic, metal (e.g., stainless steel, aluminum, etc.) or preferably, any other nontransparent material. Further, the optical filters 132 are arranged so that an optical beam is partially transmitted and partially reflected by each optical filter, in sequence, producing a cascading (zig-zag) light path.
One significant problem associated with the prior art optical multiplexing devices 200 (shown in FIG. 1) having optical blocks 120 is the expense associated with precisely machining a pair of opposite sides 112 and 114 of an optical block 120, so that the optical filters 132 that are attached to the sides 112 and 114 of the optical block 120 can be mounted and aligned in nearly perfect parallelism to the optical block 120. The prior art design of the optical block 120 had a relatively large area, namely, the sides 112 and 114 that required an optical (mirror) finish. The mirror finish was required in order to mount the filters 132 flat against the optical block 120 in order to maintain parallelism. The relatively large surface area 112 and 114 was required on the optical block 120 because this served as the gluing contact area for the filters 132. It is difficult to maintain a large surface without any pits, scratches or dust to the point that a completely flat surface is maintained. Any surface anomaly tilted the filter out of parallel. To overcome this problem, a microsphere solution was applied to even out the mirror surface during the filter mounting process. Parallelism of the filters 132 to the mounting surface is critical to the optical performance of the optical multiplexing assembly, since it is presumed that every channel will match a specified center wavelength for its transmission bandpass at the same angle of incidence (AOI). The parallelism is measured, typically using an interferometer, and is kept to within 5 fringes at a wavelength of about 650 nm, that is, to within 0.03 degrees relative to the optical block surface. The effect of deviation from parallelism can accumulate as the beam of light travels from one filter to another. For instance, in an assembly consisting of five primary filters, the effect of deviation can result in an AOI error of approximately 0.2 degrees on the last filter in the assembly. Moreover, since the filters are quite small, generally being on the order of 1 to 5 mm in cubic size, difficulties in handling the filters and in precisely mounting the filters onto the optical block, can be time consuming and costly given the uncertainty as to the precise wavelength of a manufactured optical filter. Furthermore, improper mounting of the filters can significantly decrease the optical accuracy and thermal stability of the device.
A related problem of optical multiplexing devices 200 is the gluing of the filters 132 to the respective opposite sides 112 and 114 of the optical block 120 (shown in FIG. 1), where a thermally cured epoxy is applied to the interface between each of the filters 132 and the sides 112 and 114 of the optical block 120. Since the bandpass film in the filters 132 faces the sides 112 and 114 of the optical block 120 containing the optical slot 108, the epoxy tends to interfere with the path of the optical signal, thus, resulting in system degradation. Another limitation of such an optical multiplexing device 200 design is the difficulty in cleaning the bandpass filter 132 surfaces after the filters 132 are attached to the sides 112 and 114 of the optical block 120. Additionally, the precise mounting and gluing of the individual optical filters 132 to the optical block 120 tends to be a lengthy process. In particular, the filters 132 are first attached to one side 114 of the optical block 120 and then the optical block 120 is baked at 60 degrees Celsius for two hour, which secures the respective filters 132 to that one side 114 of the optical block 120. The filters 132 are then inspected for parallelism and flatness to the optical block 120, and any excess glue is cleaned off the filters 132. Next, additional optical filters 132 are mounted onto an opposite side two 112 of the optical block 120 and then the optical block 120 is again baked at 60 degrees Celsius for two hours, which secures the respective filters 132 to that side two 112 of the optical block 120. The filters 132 are then inspected for parallelism and flatness to the optical block 120, and the filters 132 are also cleaned. The optical block 120 is then mounted onto the substrate 125. The optical block 120 and the substrate 25 are baked at 120 degrees C. for another hour. Thus, the curing time can take about five hours of the manufacturing cycle time.
A problem with the design of optical multiplexing devices 200 is that such devices employ optical blocks that are all the same size and, thus, the optical blocks can only accommodate optical filters that are optimized at one tilt angle, for instance, about 5.0 degrees, whereas, the tilt angle at which the collimated light 118 enters the optical block 120 can vary from about 6 to 8 degrees. In particular, the wavelength of light passed by each filter is highly sensitive to two parameters: 1) the angle of incidence between the filter surface and the light beam, and 2) the position of the beam on the filter""s surface. To accommodate this design requirement, all filters in one WDM device are pre-selected to function as a xe2x80x9ckitxe2x80x9d, that is, each of the filters operates at one common angle of incidence with respect to each of the respective filter""s geometric center point. Accordingly, for such optical multiplexing devices 200, it is not physically possible to hit the geometric center of each of the filters at the proper AOI that is optimized at any angle other than 5.0 degrees.
Another problem associated with the manufacturing of optical multiplexing devices is the finding an optical block material that matches the filters"" coefficient of thermal expansion (CTE), which can entail a lot of work, and is not always a success. Further, the optical multiplexing devices tend to be sensitive to polarization dependent losses (PDL) during thermal cycles. In a design where the optical block is bonded at the sides to the thin film coating of the filter, the thin film coating can be extremely sensitive to non-uniform stress induced by the thermal expansion mismatch between the coating and the epoxy as well as that of the material used for the optical block, thus, resulting in high PDL failures.
In light of the foregoing, it is desirable to provide an optical multiplexing device that is cost-efficient without sacrificing reliability in performance. Also, it is desirable to simplify the manufacturing and testing process for optical multiplexing devices and, also, increase line production and reliability with minimal rework. Further, it is desirable to provide a method for making an optical multiplexing device that is overall cost-efficient.
The present invention is directed to a blockless optical multiplexing device that substantially obviates one or more of the limitations and disadvantages of the related art. The present invention provides a solution to the problems described above relating to the manufacture of optical multiplexing devices. Specifically, the invention provides a blockless optical multiplexing device with N number of different optical channels and a method of making such a blockless optical multiplexing device with N number of different optical channels on one manufacturing line, N being equal to or greater than 2.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described, the invention provides a method of mounting N optical filters onto a substrate, where Nxe2x89xa72, preferably, N=4, more preferably, N=8, and most preferably, N=16. The method comprises the steps of providing a holding fixture for holding the substrate in a vertical position for assembling each of the N optical filters onto the substrate. The method further comprises removably attaching an alignment fixture onto the substrate, abuttingly aligning each of the N optical filters to the alignment fixture, and securing each of the N optical filters to the substrate, where each of the N optical filters is precisely aligned in parallel with each other. The dimensions of the alignment fixture are customized for the specific angle of incidence of each WDM device to be manufactured. Thus, an array of variously dimensioned alignment fixtures are provided for accommodating each possible angle of incidence for each filter kit employed. The securing step includes abuttingly aligning a first of the N optical filters against a first side of the alignment fixture, where the first of the N optical filters is precisely aligned to directly receive, at a geometric center, a collimated beam of multi-wavelength light at a desired angle of incidence. The securing step further includes applying a curable adhesive to secure a bottom surface of the first of the N optical filters to the substrate, checking the parallelism of the first of the N optical filters to the alignment fixture and curing the curable adhesive. The securing step further includes abuttingly aligning a second of the N optical filters against an opposite second side of the alignment fixture and in parallel with the first of the N optical filters, and applying a curable adhesive to secure a bottom surface of the second of the N optical filters to the substrate, checking the parallelism of the second of the N optical filters to the first of the N optical filters, and curing the curable adhesive. The securing step further includes abuttingly aligning a third of the N optical filters against the first side of the alignment fixture and in parallel with each of the first and second of the N optical filters, applying a curable adhesive to secure a bottom surface of the third of the N optical filters to the substrate, checking the parallelism of the third of the N optical filters to each of the first and the second of the N optical filters, and curing the curable adhesive. The securing step further includes abuttingly aligning a fourth of the N optical filters against the second opposing side and in parallel with each of the first, second and third of the N optical filters, applying a curable adhesive to secure a bottom surface of the fourth of the N optical filters to the substrate, checking the parallelism of the fourth of the N optical filters to each of the first, second and third of the N optical filters, and curing the curable adhesive. Finally, the method further comprises the step of removing the alignment fixture from the substrate, whereby an air gap is created between one half of the N optical filters and a second half of the N optical filters. In a preferred embodiment, the removably mounting step includes removably mounting the alignment fixture onto the substrate with a fastener or a bolt. The method also includes selecting an alignment fixture that is suitable for aligning each of the N optical filters within a desired angle of incidence, and where the first side and the second opposing side of the alignment fixture are substantially parallel to each other. In addition, the checking step includes the step of checking the parallelism of each of the N optical filters with an instrument, preferably, an interferometer. Also, in a preferred embodiment, each of the respective first and third of the N optical filters are abutted against and parallel to each other on one side of the air gap, and wherein each of the respective second and fourth of the N optical filters are abutted against and parallel to each other on an opposite side of the air gap, with the number N of optical filters being equal to the number of optical channels desired. Moreover, in a preferred embodiment, the curable adhesive is an epoxy and each of the curing steps includes exposing the substrate to a high intensity ultra-violet light for preferably approximately 30 seconds.
In another aspect, the invention provides a method of assembling a blockless optical multiplexing device having N sub-assemblies, where Nxe2x89xa71. The method comprises the steps of removably attaching an alignment fixture onto a substrate of each of the N sub-assemblies, precisely aligning each of a plurality of optical filters in parallel with each other and abutting against either one of two opposite sides of the alignment fixture of each of the N sub-assemblies, preferably, with one half of the plurality of optical filters abutting against one side of the alignment fixture and with the other half of the plurality of optical filters abutting against the other side of the alignment fixture. The method further comprises securing each of the plurality of optical filters to the respective substrate of each of the N sub-assemblies, where each of the plurality of optical filters are precisely aligned in parallel with each other on the respective substrates of each of the N sub-assemblies. The method further comprises the step of removing the alignment fixture from the substrate of each of the N sub-assemblies, such that an air gap is created in between the plurality of optical filters where the alignment fixture originally was attached. The method further comprises the steps of fastening, to the substrate of each of the N sub-assemblies, an input collimator that is aligned to transmit a collimated beam of multi-wavelength light to a first one of the plurality of optical filters, and affixing each of a plurality of output collimators to the substrate of each of the N sub-assemblies, where one of the plurality of output collimators is aligned with respect to an associated one of the plurality of optical filters, preferably, each of the output collimators is positioned behind each of the associated filters. In addition, the method comprises mounting each of the N sub-assemblies onto a main assembly unit, wherein the output end of a preceding one of the N sub-assemblies is optically connected to the input end of a succeeding one of the N sub-assemblies, such that a collimated beam of multi-wavelength light can be transmitted through the air gap and through a geometric center of each of the plurality of optical filters in a cascading path on each of the N sub-assemblies of the optical multiplexing device. In a preferred embodiment, the removably attaching step includes the steps of providing a holding fixture for holding the substrate of each of the N sub-assemblies in a vertical position and further selecting an alignment fixture that is suitable for aligning in parallel each of the N optical filters, such that the collimated beam of multi-wavelength light is incident to the geometric center of each of the plurality of optical filters at a desired angle of incidence. In a preferred embodiment, the desired angle of incidence for each of the filters is the same. Further, in a preferred embodiment, the precisely aligning step includes the steps of applying a curable adhesive to secure a bottom surface of each of the optical filters to the respective substrate, and measuring the parallelism of each of the optical filters in relation to each other, preferably, with an interferometer. Further, in a preferred embodiment, the securing step includes the step of curing the curable adhesive, preferably, by exposing each of the respective substrates to a high intensity ultra-violet light for approximately 30 seconds. In a preferred embodiment, the number N of sub-assemblies in the optical multiplexing device is 4, where each of the 4 sub-assemblies comprises, among other optical components, preferably, 4 optical filters, 1 input collimator and 4 output collimators, thus, providing a blockless optical multiplexing device having a total of, preferably, 16 optical channels.
In a further embodiment, the invention provides a method of making a thin film optical telecommunications multiplexing device having N optical channels, where N is equal to or greater than 2. The method comprises the steps of temporarily attaching an alignment fixture onto each one of a plurality of substrates. For each one of the plurality of substrates, abuttingly aligning one of a plurality of optical filters against one of two opposite sides of the alignment fixture and measuring the angle of incidence that an incident beam of test light makes with the one of the plurality of optical filters on each of the substrates. If a desired angle of incidence is not present, making adjustments to the alignment of the one of the plurality of optical filters. However, if the desired angle of incidence is present, permanently securing the one of the N optical filters to each of the respective substrates. For each of a subsequent one of the plurality of optical filters, repeating the steps of abuttingly aligning the subsequent one of the optical filters against the alignment fixture and measuring the angle of incidence that an incident beam of test light makes with the subsequent one of the plurality of optical filters on each of the substrates. If a desired angle of incidence is not present, making adjustments to the alignment of the subsequent one of the plurality of optical filters. However, if the desired angle of incidence is present, permanently securing the subsequent one of the N optical filters to each of the respective substrates, such that one-half of the plurality of optical filters abut against the one of the two opposite sides of the alignment fixture and the other half of the plurality of optical filters abut against the other of the two opposite sides of the alignment fixture. In a preferred embodiment, for each of the plurality of substrates, the abuttingly aligning step includes the steps of applying a curable adhesive to secure a bottom surface of each of the plurality of optical filters to a respective one of the plurality of substrates. Further, each of the plurality of substrates, the permanently securing step includes the step of curing the curable adhesive by exposing the respective substrates to a 30-second high intensity ultra-violet light. The method further comprises removing the alignment fixture from each of the plurality of substrates after each of the plurality of optical filters are secured to the respective substrates and securing an input collimator onto each of the substrates, where the input collimator is precisely aligned to direct a collimated multi-wavelength light at the desired angle of incidence. In addition, the method includes securing a plurality of output collimators onto each of the substrates, where a respective one of the output collimators is aligned with respect to an associated one of the optical filters, such that the collimated beam of the multi-wavelength light can be transmitted through an air gap and through a geometric center of each of the N optical filters in a cascading path on each of the plurality of substrates. The method further comprises mounting each one of the substrates onto a larger or main substrate, where an output end of a preceding one of the substrates is optically connected to an input end of a subsequent one of the substrates. In a preferred embodiment, the number of optical filters and the number of output collimators on the substrates is equal to the number of optical channels N making up the multiplexing device, preferably, N=4, more preferably N=8, and most preferably, N=16. In a preferred embodiment, each of the substrates have an equal number of optical filters and output collimators, and where the total number of optical filters is equal to the total number of output collimators, which in turn dictates the total number N of optical channels in the multiplexing device.
In yet another embodiment, the invention provides a blockless optical multiplexing device. The blockless optical multiplexing device comprises at least one unit, preferably two units, more preferably three units, and most preferably four units. Each of the units comprises a substrate, an input collimator mounted onto the substrate, a plurality of optical filters precisely secured onto the substrate and a plurality of output collimators mounted onto the substrate, with one half of the optical filters being separated by an air gap from a second half of the optical filters and, with a respective one of the output collimators being aligned and associated with a respective one of the optical filters. In a preferred embodiment, a collimated beam of wavelength light is transmitted through a geometric center of each of the optical filters in a cascading path through the air gap.